Why are some user name formatted user########, is this linked with unregistered users and if does a unregistered status appear on every unregistered users profile?
You can see whether a user is registered or not in their profile. Unregistered users have "unregistered" right beside their name.
Here's an example from Unix & Linux:
That's an unregistered user that chose a username. You can find plenty of registered users on Stack Overflow that didn't change their display name from the default